For many types of end-stage organ failure, transplantation has become the treatment of choice. The major source of morbidity associated with this treatment is host rejection of the transplanted organ. The long term goal of this program project is to prolong graft survival in transplant patients. Our approach is to decrease the ability of the graft to trigger the host immune system, in contrast to the more common approach involving immunosuppression of the host. We propose to develop a series of oligonucleotides designed to down-regulate the expression of MHC class I and class II genes, whose products play a crucial role in the immune response. Our working hypothesis is that depletion of MHC antigens will correspondingly reduce the ability of graft cells to excite the host immune response. The oligonucleotides under consideration are based on several different modes of biological action: antisense, antigene, aptamer and decoy. Our program is organized around a three-stage development process for the rational design of therapeutic oligonucleotides: (i) synthesis and modification; (ii) structural characterization; and (iii) evaluation of cellular and immunological effects. In addition, we propose to develop several promising oligonucleotides that will serve as paradigms for future developments. Covalent modifications of oligonucleotides will be developed to enhance their uptake, resistance to nucleases, and delivery to their targets. The structure and stability of oligonucleotide complexes will be investigated by spectrophotometric, electrophoretic, calorimetric and NMR methods. Results from these studies will help interpret results from cellular studies in terms of mechanism of action. In particular, we will determine the extent to which constitutive and induced expression of MHC genes can be reduced by therapeutic oligonucleotides and the resultant effect on the immune response.